This invention relates generally to a method and apparatus for enhancement of artificially cultured agricultural products within an environmentally controlled greenhouse environment. More particularly, the invention relates to utilization of waste gas from a landfill to provide enhanced crop growth within a greenhouse in an environmentally safe and controlled system.
Yields from food crops and lengthening of the growing season can be achieved by growing the crops under environmentally controlled conditions in large greenhouses. For example, yields of tomatoes grown in a hydroponic greenhouse with a computer controlled growing environment may be many times higher than the yield of tomatoes grown outdoors which are subject to a shorter growing season and uncontrolled environment. Such a greenhouse is described in an article in the Business section of The Dallas Morning News dated Sunday, Jan. 12, 1997 entitled "High-Tech Tomatoes". The tomatoes are grown in a hydroponic system, supported not in soil but in a porous artificial "rock wool" made from volcanic material. Correct amounts of water and nutrients are supplied to the plants and the plants are kept warm by a circulating hot water system heated by burning natural gas (methane). Controlled amounts of exhaust gas from the burners containing carbon dioxide (CO.sub.2) is recycled back into the greenhouse.
It is well known that carbon dioxide (CO.sub.2) is an essential ingredient of the process of photosynthesis, and will enhance plant growth.
Photosynthesis is a metabolic process, occurring in green plants, and characterized by the utilization of light (usually sunlight) to convert carbon dioxide and water into carbohydrates and oxygen. The chemical formula for the photosynthetic reaction is: EQU 6CO.sub.2 +6H.sub.2 O+sunlight=C.sub.6 H.sub.12 O.sub.6 +6O.sub.2
Most green plants of the Temperate Zone photosynthesize at temperatures between 10.degree. C. and 35.degree. C. (50.degree. F.-95.degree. F.). The most important external factors are: intensity and duration of light; concentration of available carbon dioxide; temperature; and water supply. Sunlight of great intensity and long duration increases the amount of photosynthesis performed. About 0.03 of one percent of the total volume of the atmosphere is made up of carbon dioxide; hence, one of the limiting factors on the extent of photosynthesis is the amount of available carbon dioxide.
When plants are artificially cultured these external factors can sometimes be controlled. While artificial cultivation of crops in a controlled greenhouse 20 environment increases the yield, it is also a more expensive process because of the requirement to burn natural gas or other fuel to warm the crops and also to enhance the ambient CO.sub.2 levels in the greenhouse which can be accomplished by injection of expensive purchased CO.sub.2, or, utilization and injection of the CO.sub.2 rich flue gas from the combustion of the natural gas fired heat generators. On the other hand, a potential energy source is available from municipal solid waste landfills, which are generally conceded to be an environmental liability and source of atmospheric pollution.
Each person in the United States generates about 4.5 pounds of waste per day, or almost one ton per year, most of which is deposited in municipal solid waste landfills. As this land filled waste decomposes, it produces landfill gas. Uncollected landfill gas contributes to the formation of smog and poses an explosion hazard if uncontrolled. Collected and controlled landfill gas is a viable source of energy. Landfill gas is typically a medium Btu gas that has a number of energy applications. The most prevalent use is production of electricity for sale to a local utility via turbines or engines. Other options include direct use of the gas as boiler fuel, production of compressed natural gas for vehicle fuel, or upgrade to pipeline quality gas.
On the negative side, landfill gas is harmful to the environment because it is primarily made up of so-called "greenhouse gases" which contribute to global warming. The "greenhouse gases" capture heat from the sun-warmed earth, which would otherwise be rechanneled back into space, contributing to the so-called "greenhouse effect". The "greenhouse gases" are carbon dioxide, methane, nitrogen oxide, chlorofluorocarbons and ozone. Landfill gas typically contains 30%-55% methane and 25%-45% carbon dioxide and trace amounts of non-methane hydrocarbons (NMO), chloro-hydrocarbons, and reduced sulphur compounds (TRS)/(H.sub.2 S).
Regulations already require many landfills to collect their landfill gas emissions. Once the gas is collected, landfill owner/operators have two choices: (1) flare the gas; or (2) produce energy for sale or on-site use. Both options address local air quality and safety concerns, but only energy recovery capitalizes on the energy value of landfill gas and displaces the use of fossil fuels.
Various proposals have been made for utilization of landfill gas. A process for producing essentially pure carbon dioxide from landfill gas is described in U.S. Pat. No. 5,059,405 issued Oct. 22, 1991 to Watson et al. This patent describes a process and apparatus for removing the impurities from a gas stream produced from a landfill such that essentially pure carbon dioxide and methane is recovered. After the landfill gas is mechanically dewatered, the gas is filtered of particulate solids and aerosols and purified by removing sulfur compounds using zinc oxide columns, removing halogens using activated alumina columns, removing hydrocarbons using activated charcoal columns, and oxidizing remaining impurities using potassium permanganate impregnated activated alumina columns. Lastly the gas is incinerated in a boiler/incinerator combustion furnace to produce an exit stream containing essentially pure carbon dioxide and air, which is further treated in a conventional carbon dioxide treatment process.
The CO.sub.2 stream resulting from the process is compressed for storage in a liquified condition in a storage tank. According to the patent, the starting landfill gas stream typically contains 40%-60% CH.sub.4, 30%-45% CO.sub.2, 1%-3% O.sub.2, 3% to 10% N.sub.2, by volume and various trace impurities.
Another process for utilizing landfill gas is described in U.S. Pat. No. 5,451,249 issued Sep. 19, 1995 to Spiegel et al. The gas stream which emanates from landfills is treated to produce a purified gas which is essentially a 20 hydrocarbon such as methane which can be used as the fuel source in a fuel cell power plant, or can be used in other power plants which use natural gas as a fuel. The landfill gas passes through a system which removed essentially all of the hydrogen sulfide; water; organic sulfur and halogen compounds; and solid contaminants from the gas stream. The resultant purified gas stream can be cleanly flared; used to power an energy plant; or put to other useful purposes. The resulting fuel is used in a fuel cell operated on landfill gas at a landfill site.
Where landfill gas is emitted into the atmosphere without recovery and use, the carbon dioxide and methane have a global warming potential. Furthermore some of the non-methane constituents of landfill gas such as hydrogen sulfide (H.sub.2 S), are odoriferous and also potentially harmful to the environment. In the fuel cell process, moisture and volatile organic compounds (VOCs) including sulfides and halogenated compounds are removed. Dual clean up trains are employed in parallel absobers, so that when one train is in service cleaning the gas, the other is being regenerated with a portion of the cleaned gas.
In the foregoing patented systems, the total flow stream of landfill gas is processed, either to produce pure CO.sub.2 for industrial purposes, or to produce a purified hydrocarbon gas to be used as a fuel source in a fuel cell or power plant.
It would be desirable to integrate the needs of an environmentally controlled greenhouse for growing plants so as to utilize the environmentally damaging landfill gas emanating from a municipal solid waste landfill in a manner which is both economically beneficial and environmentally safe, so as to increase the yield of growing plants while reducing the emission of potentially polluting gas to the atmosphere.
Accordingly, the primary object of the invention is to provide an improved method and apparatus for utilizing landfill gas to enhance plant growth in a greenhouse.